I can recommend Andrews & Arnold at £1.44 per month for VOIP. Not sure if they do redirect to mobile. I have a Grandstream HT-801 ATA (Analog Telephone Adapter) so I can continue using old pulse dialling gear and familiar answering machines etc.
Are you a member of the RSGB ? They have an experienced team on the QRM helpdesk and solar inverters are a big issue - they have sometimes helped. If you aren’t in the UK perhaps your national amateur radio society has a similar resource?
The capacitors do not provide a safety function, on the contrary they only increase hazard. They are there purely for EMI/RFI interference reasons - which are important too. If the live and neutral from the inverters have no relationship to ground then any downstream RCD/RCBO/GFCI (USA speak)...
I know for a fact that the xpelair DX100T fan worls fine from a dimmer live. Even the lowest dimmer setting where filaments barely glowing (this installation had incandescent lights) was sufficient to trigget the fan and overrun time was same as when triggered by higher dimmer settings.
Thermomix is German so the designers may be more used to their 220V end of the nominal 230V mains and set the alarm trigger maximum voltage to what they thought adequate but which is too low for the UK? The UK is around the 240V end of the nominal range and allowed to go 254V and that can...
You can sometimes get incompatible led lights and dimmer to cooperate by having a small filament bulb in the system. You could try 7 LED and 1 filament/halogen?
Can you just not use 13A domestic type sockets and instead a non-standard socket strip like IEC or Schuko or CEE etc etc then Mrs Mop can't plug in the kettle and knock out the server gear?